A proximity and contact sensor has been developed for equipment such as medical equipment used for diagnosis or treatment purposes. A proximity and contact sensor detects whether a human body approaches to the equipment in a non-contacting method. A conventional proximity and contact sensor includes a detection plate formed by two electrodes and detects the approaching of a human body through changes in the capacitance of the detection plate (refer to patent document 1). The conventional proximity sensor consists of an oscillation circuit, which generates a measurement signal having a predetermined frequency, a VSWR bridge circuit, which is connected to the oscillation circuit, a phase comparison circuit, which detects the phase difference between an output signal of the bridge circuit and the measurement signal of the oscillation circuit, and a control method, which determines whether or not a human body is approaching based on the output of the phase comparison circuit.
Another conventional proximity sensor consists of a sensing part of a capacitance type proximity and contact sensor and a conductor, in which a thickness and a surface resistance of the conductor are set in a predetermined range (refer to patent document 2). In this proximity and contact sensor, the ratio between the conductor area and the sensing part area of the capacitance type proximity and contact sensor is adjusted to be two times or higher value so as to achieve both of limiting the enlargement of the sensing area and widening the detection range.    [Patent Document 1] Japanese Laid-Open Patent Publication No. 2001-203565    [Patent Document 2] Japanese Laid-Open Patent Publication No. 2004-150869